Help with dealing with a creaking sound

Hello, I’m relatively new to cycling and have been going on rides with my dad’s Cannondale CAAD 8 that he doesn’t use anymore.

I’ve taken it into my local bike shop for some work and have been trying to learn basic maintenance. A few weeks ago, I noticed this creaking noise when I pedal. Sometimes it happens if I’m coasting as well. I’ve taken it to the shop for help, he told me about how Shimano’s shifters can “trim.” I told him that I don’t think that’s it but he insisted. Went for a ride after and, of course, it’s still happening.

Any help or guidance would be greatly appreciated. Here is a video I took during my last ride that has the sound.

https://imgur.com/a/2JeBaTe

Thanks!